We use Total Traffic Control (TTC) from Lightspeed Systems: http://www.lightspeedsystems.com/. It is run on a Win2003 box. It is almost like an appliance in the sense that they have written their own TCP stack that replaces the standard MS TCP/IP stack.
They are mostly geared toward edu markets, but I hear there are several enterprise environments that are starting to pick them up. We also run TTC for web filtering, though that is a separate box. Mike ~ Upgrade to Next Generation Antispam/Antivirus with Ninja! ~ ~ <http://www.sunbelt-software.com/SunbeltMessagingNinja.cfm> ~
